audio_format: renamed sampleRate to sample_rate

The last bit of CamelCase in audio_format.h.  Additionally, rename a
bunch of local variables.

src/crossfade.c

@@ -37,7 +37,7 @@ unsigned cross_fade_calc(float duration, float total_time,
 	assert(duration > 0);
 	assert(af->bits > 0);
 	assert(af->channels > 0);
-	assert(af->sampleRate > 0);
+	assert(af->sample_rate > 0);
 
 	chunks = audio_format_time_to_size(af) / CHUNK_SIZE;
 	chunks = (chunks * duration + 0.5);